This invention is concerned with a method of melting titanium aluminide alloys in ceramic crucibles.
The melting of small quantities of titanium was first experimented with in 1948 using methods such as resistance heating, induction heating, and tungsten arc melting. However, these methods never developed into industrial processes. The development during the early 1950s of the cold crucible, consumable-electrode vacuum arc melting process, known as "skull melting," by the U.S. Bureau of Mines made it possible to melt large quantities of titanium with minimal contamination into ingots or net shapes.
Titanium aluminide alloys are made by arc melting under protective conditions, for example, in an inert atmosphere such as argon, in a water cooled copper crucible by the skull melting process. Briefly described, vacuum arc skull melting furnaces consist of a vacuum-tight chamber in which a titanium or titanium alloy electrode is driven down into a water-cooled copper crucible. A dc powder supply provides the fusing current needed to strike an electric arc between the consumable electrode and the crucible. Because the crucible is water cooled, a solidified skull of the titanium or titanium alloy melt forms at the crucible surface, thus avoiding direct contact between melt and crucible. Once the predetermined amount of liquid titanium is contained in the crucible, the electrode is retracted, and the crucible is tilted to pour the melt into a casting mold positioned below. Special containers such as water cooled copper crucibles are required to melt refractory metals because of the strong reactivity of refractory metals, such as titanium, with ceramic crucibles.
Although the skull melting process is a proven and capable method for melting titanium and titanium alloys, it is energy intensive and affords little opportunity for superheating the molten metal because of the cooling effect of the water-cooled crucible. Because of the limited superheating, it is common to either pour castings centrifugally, forcing the metal into a mold cavity, or to pour statically into preheated molds to obtain adequate fluidity. It is highly desirable to develop methods for melting titanium alloys in ceramic crucibles to reduce the energy required for melting, and allow for obtaining higher levels of superheating. However, the ceramic crucible melting must provide a level of oxygen pickup in the melt that is comparable to the oxygen pickup achieved in the skull melting process.
The titanium alloys of interest for melting in the method of this invention are the gamma titanium aluminides. Gamma titanium aluminides are well known being characterized by a tetragonal crystal structure, and are comprised of about 48 to 58 atom percent aluminum. Gamma titanium aluminide alloys comprised of a minor amount of alpha-2 phase are comprised of as low as 40 atom percent aluminum. Additional elements, for example, chromium, vanadium, niobium, tantalum, silicon, and gallium have been added to gamma titanium aluminide alloys as shown for example in U.S. Pat. Nos. 3,203,794; 4,294,615; 4,661,316; 4,857,268; 4,842,820; 4,842,817; 4,836,983; 4,879,092; 4,902,474; 4,897,127; 4,923,534; 4,916,028; incorporated herein by reference. The low ductility of the gamma titanium aluminides at room temperature has been the major limitation to forming components of the alloys. It is well known that oxygen is an interstitial contaminant in gamma titanium aluminides that contributes to the room temperature brittleness of the alloy.
It is an object of this invention to provide a method for melting gamma titanium aluminide alloys in a ceramic crucible, while minimizing oxygen pickup in the melt.